This invention relates to high voltage, high frequency transistor power amplifiers and, more particularly, to high frequency amplifiers which utilize series connected transistors to permit high operating voltages.
Transistor operating voltages are limited by inter-region breakdown voltages, for example, BVCES (collector-to-emitter breakdown voltage) in the case of a bipolar transistor and BV.sub.DSS (drain-to-source breakdown voltage) in the case of a field-effect transistor. When a field-effect transistor is used as the active component in a class C amplifier and the output impedance of the transistor is properly matched, the transistor drain voltage normally swings from the drain-source saturation voltage to approximately twice the supply voltage. Similar voltages occur in class C amplifiers using bipolar transistors. If the load impedance is not matched to the output impedance of the transistor, voltage standing waves with peak amplitudes greater than twice the power supply voltage are possible. Therefore, supply voltages somewhat lower than one-half the maximum breakdown value are normally recommended.
Present day power transistors that are designed to operate at frequencies above 100 megahertz are generally limited to dc power supply voltages below 36 volts.
These supply voltage levels have been established as standards of the industry, probably for reasons ranging from safety to availability of portable power supplies in the form of batteries.
In order to increase output power, parallel active devices or parallel power amplifiers are commonly utilized. The drive power is divided and applied to the individual amplifying devices and the output powers are then combined to achieve the desired power level. This produces a new higher power amplifier which operates at the same dc supply voltage but draws more dc current depending on the number of paralleled power units.
An alternate, and in some cases a more desirable, approach to high frequency, high power amplification is to utilize a higher supply voltage. This approach takes advantage of the relatively high ac voltage available in most homes, offices, and industrial buildings. Conventionally, ac line voltages are stepped down by transformers and rectified to dc voltages compatible with transistor operation. Elimination of power transformers with their associated cost and weight is a major advantage of operating power amplifiers at high voltages. Furthermore, the current requirements are reduced when the supply voltage is increased and the requirement for high current conductors is correspondingly reduced.
Ideally, transistors having high breakdown voltage capabilities would be utilized with high supply voltages. However, high breakdown voltage and high frequency operation are conflicting requirements in semiconductor device design. Because of the thin interaction regions necessary to achieve high operating frequencies, these devices have inherently low breakdown voltages.